It is known to connect together mating inner and outer components using a tolerance ring. For example, a tolerance ring may be used to hold a shaft in place in a corresponding bore, or it may act as a force limiter to permit torque to be transmitted between the shaft and the housing. The use of a tolerance ring accommodates minor variations in the diameter of the inner and outer components without substantially affecting their interconnection.
Typically, a tolerance ring comprises a band of resilient material, e.g. a metal such as spring steel, the ends of which are brought together to form a ring. A strip of projections extend radially outwards from the ring, or radially inwards towards the centre of the ring. The projections can be formations, possibly regular formations, such as corrugations, ridges, waves or fingers.
In use, the tolerance ring is located between the components, e.g. in the annular space between the shaft and the bore in the housing, such that the projections are compressed. Each projection acts as a spring and exerts a radial force against the components, providing an interference fit between them. Rotation of the inner or outer component will produce similar rotation in the other component, as torque is transmitted by the ring. Likewise, linear movement of either component will produce similar linear movement in the other component, as linear force is transmitted by the ring.
If forces (rotational or linear) are applied to one or both of the inner and outer components, such that the resultant force between the mating components is higher than a certain threshold value, the inner and outer components will move relative to one another, i.e. they will slip.
Although tolerance rings usually comprise a strip of resilient material that is curved to allow the easy formation of a ring by overlapping the ends of the strip, they may also be manufactured as an annular band.
During assembly of an apparatus with an interference fit between components, a tolerance ring is typically held stationary with respect to a first (inner or outer) component whilst the second component is moved into mating engagement with the first component, thereby contacting and compressing the projections of the tolerance ring to provide the interference fit. The amount of force required to assemble the apparatus may depend on the stiffness of the projections and the degree of compression required. Likewise, the load transmitted by the tolerance ring in its final position and hence the amount of retention force provided or torque that can be transmitted may also depend on the size of the compression force and the stiffness and/or configuration of the projections.
One example of the use a tolerance ring is in a hard disk drive pivot mount, where the tolerance ring provides axial retention between a rotatable pivot shaft and an arm mounted thereon. These components are delicate, which imposes constraints on the size of assembly force and compression forces that can be used.
Furthermore, the coupling between the mating components may exhibit resonant behaviour, i.e. where external vibrations are amplified in the coupling. The resonant frequency or frequencies of an assembly are important in determining the operation of that assembly. For example, in hard disk drive pivot mounts accurate data writing cannot take place when resonance occurs, so it is important to know the frequency of resonance. The resonant frequency may depend on amount of compression that takes place during installation, i.e. may depend on an assembly force.